Short answer. An unpaid seller has four remedies even after ownership passes to the buyer: a lien on the goods while he still holds them, the right to stop them in transit if the buyer becomes insolvent, a right of resale, and a right to rescind the sale. If ownership has not passed, he may also simply withhold delivery.
What the law says
notwithstanding that the ownership in the goods may have passed to the buyer, the unpaid seller of goods, as such, has: (1) A lien on the goods or right to retain them for the price while he is in possession of them
Civil Code, Article 1526 — Rights of an Unpaid Seller. Read the full provision →
Why the remedies survive the transfer of ownership
The point that catches sellers by surprise is the opening clause. These rights exist notwithstanding that the ownership in the goods may have passed to the buyer. In a sale of goods, title can pass on perfection or on delivery depending on the terms, and many sellers assume that once title has moved they are reduced to suing for the price like any ordinary creditor. Article 1526 says otherwise. The law gives the unpaid seller real rights over the goods themselves, which is a far stronger position than an unsecured money claim — especially against a buyer who is running out of cash.
Lien, stoppage, resale, rescission
The lien is a right to retain the goods for the price while they are still in your hands; you are not obliged to hand them over and then chase payment. Stoppage in transitu lets you intercept goods already shipped, but only in case of the insolvency of the buyer — it is an insolvency remedy, not a general change-of-mind remedy. The right of resale lets you sell the goods to someone else and look to the buyer for the shortfall. The right to rescind unwinds the sale altogether. The last two are expressly "as limited by this Title", meaning the Civil Code's own conditions on notice and timing govern how they are exercised.
Where ownership has not passed
If title is still with the seller, the Code adds a fifth tool: a right of withholding delivery, described as similar to and coextensive with the lien and stoppage rights. In practice this means a seller who has not yet parted with either title or possession is in the strongest position of all. It also means the answer to "can I just keep the goods?" turns on facts that sellers rarely document carefully — when title was agreed to pass, whether delivery to a carrier counted as delivery to the buyer, and who bore the risk in transit. Those terms belong in the contract, in writing, before a dispute starts.
The limits, and what it costs to get them wrong
These are remedies over goods — movable property in a sale of goods — not a general licence to seize a debtor's assets, and not a tool for a services contract or a sale of land. Exercising them wrongly is expensive: retaining goods you have no lien over, or reselling when the conditions for resale are not met, can turn you from the aggrieved party into the party in breach and expose you to damages. Third parties complicate matters further, because a buyer who has already resold to someone acting in good faith may have put the goods beyond your reach. Before stopping a shipment or reselling, have a lawyer read the contract and the shipping documents.